Mercaptan extraction



Patented Aug. 19, 1947 Chester E. Adams, Highland,

Tom, Oil Company, Indiana Hammond, Ind., assignors and Theodore H. i

to Standard Chicago, 111., a corporation of No Drawing. Application August 2.2, 1945, Serial No. 612,126

Claims. 1

This invention relates to the extraction of weakly acidic substances from hydrocarbon solutions and more particularly it relates to the extraction. of mercaptans from hydrocarbon oils such as petroleum distillates, naptha, gasoline, kerosene, etc. In the manufacture of gasoline and other petroleum distillates known in refinery practice as light oils, it has long been a problem to remove therefrom ill-smelling sulfur compounds which are primarily mercaptans. Certain processes such as the doctor treating process have been employed to convert mercaptans into other sulfur compounds such as disulfides which are less objectionable from the standpoint of odor. However, this conversion does notdecrease the sulfur content of the gasoline and it has been found that the disulfides themselves are quite objectionable from the standpoint of gasoline detonation characteristics.

Numerous processes have been devised for extracting the mercaptans completely, thus producing a gasoline of higher knock rating and higher susceptibility to knock rating improvement resultin from the addition of tetraethyl lead. In general, these processes have employed strong alkali solutions in conjunction with certain mercaptan solubilizing agents known as solutizers.

One of the objects of this invention is to provide an improved solutizer for the extraction of weakly acidic substances from petroleum distillates. Another object of the invention is to provide a process of removing meroaptans more effectively from gasoline and other petroleum distillates by means of a solution which can be readily regenerated and recycled in the extraction operation. A further object of the invention is to provide a solutiz'er which is inexpensive and at the same time highly effective and which may be employed in combination with other solutizers.

According to our invention we employ as a solutizer in our process the alkali metal salts of the thioether acids which may also be termed carboxythioethers. These compounds have the following structural formula:

in which M is analkali metal, generally sodium or potassium, R1 is a hydrocarbon radical, generally alkyl, and R2 is an alkyle'ne or substituted 2 alkylene radical. We may also employ as solutizerscompounds having a, plurality of thioether radicals and/or a plurality of carboxyl groups, examples of which may be written as follows:

In this formula N is a whole number and R an alkyl radical or hydrogen, R1 is an alkylene radical and R2 is an aliphatic residue having any number of free valences depending on the value of N.

In general, we have found that the thioether acids having from 3 to ,6 carbon atoms are most effective although certain thioether acids of 8 to 10 carbon atoms may be employed, particularly when the acids are dibasic. An example of such is butyl thio succinic acid. Examples are methioacetic acid, methiopropionic acid, methiobutyric acid, eth-ioacetic acid and ethiobutyric acid. It should be understood, of course, that in actual operation the acids are present as their alkali metal salts and we have found that the potasslum salts are in general more effective than the sodium salts.

In the extraction of meroaptans from light petroleum distillates, straight-run or cracked gasoline, heavy naptha, light naptha, absorption naptha, etc, we have found that the thioether acids above described, when used in combination with caustic alkali, exhibit a high extraction efliciency as indicated by the mercaptan distribution coeflicient as hereinafter described. We have also found that the extraction efficiency is considerably increased by employing the thioether acids in combination with other well-known solutiZ/ers including the lower alcohols, the amines and certain of the organic acids, e. g. butyric acid, but we prefer to employ our new solutizer in combination with the phenols including the cresylic acids, vetc.

In carrying out our process, the hydrocarbon 5 distillate containing mercaptans or other Weakly acidic material is contacted preferably in a countercurrent extraction tower with a strong alkaline solution of the thioether acid solutizer. For this purpose, sodium or potassium hydroxide may be used, preferably the latter, in a concentration of about 2 to 6 normal free caustic above that combined with the solutizer agent. The concentration of solutizer employed will usually be within the range of about 5 to 40 per cent on the basis of free thioether acid in the alkali solution. Usually the concentration of thioether acid falls in the range of l to 3 molar. The extraction step is preferably carried out at ordinary temperature, for example 40 to 100 F,, and the solutizer solution containing dissolved mercaptans or other weakly acidic substance is withdrawn from the extractor and separately regenerated after which it is recycled to the extraction step. The regeneration operation consists in removing from the caustic solutizer solution the dissolved mercaptans or other weakly acidic substances, and is preferably carried out by blowing with air, for example at a slightly elevated temperature, e. g. 120 to 130 F. It is desirable to avoid subjecting the solutizer solution to a temperature sufiiciently high to accelerate materially hydrolysis of the thioether acid employed.

In the regeneration operation, part of the mercaptans may be removed by dissociation and evaporation and part by the oxidizing action of the air converting them to disulfides which are no longer soluble in the solutizer solution. These disulfides may be removed as an upper oily layer and, if desired, a hydrocarbon or other immiscible solvent may be employed to assist in their removal from the regenerated solutizer solution. The solutizer solution is then recycled to the continuous extraction step of the process for the accumulation of more mercaptans.

or oxygen. One very significant benefit resulting from the use of these phenolic catalysts is that they enable regeneration to be carried out at a lower temperature, for example room temperature, thereby saving the cost of heating and cooling the solution and reducing corrosion.

In the treatment of certain petroleum stocks, particularly cracked gasoline containing small amounts of phenolic compounds, especially cresols, it is desirable to precede the extraction with an alkali washing step to remove part of the phenols, cresols, etc., thus preventing an excessive amount of such phenolic substances accumulating in the solutizer solution. In the treatment of such phenol-containing stocks, however, there is a distinct advantage to permitting a controlled amount of such phenols, cresols, etc., accumulating in the solutizer solution, and We have found that the combination of cresols and our thioether acids provide a very effective reagent for extracting mercaptans. We prefer to control the amount of cresols in the range of about 1 to 3 normal concentration. Inasmuch as the distillate extracted by such a solution has been found to carry away in solution a small amount of cresols in the concentration which is in equilibrium with that in the solutizer solution, and inasmuch as cresols exert a valuable antioxidant effect on the treated gasoline, particularly in the case of cracked stocks, it is clearly desirable to employ a solutizer solution containing cresols. Other solutizers besides cresols may be employed with our thioether acids and a suitable proportion is a molar ratio to the other solutizer of 0.2 to 4.

The following table gives the results obtained in the extraction of a sour naptha. containing primarily butyl mercaptan as an example of a weakly acidic substance. Removal of the butyl mercaptan is indicated by the copper number of the naptha. Extraction was carried out in the ratio of one volume of solutizer solution to five volumes of naptha in a single batch extraction step. The composition of the solutizer solution is as indicated. The naphtha, before extraction, had a copper number of 100.

Butyl Mer- T t 4 Copper No. captan Composition of Solutizer Solution After Ex- Distribution traction Coeflicient,

5.35N KOH, 1.65N C2H5SOH2COOK 10 4.17N KOH, .83N C2H5SCH2000K, 2N K cresylate 2 245 4.35N KOH, 1.65N OzH5sCH2COOK, N K cresylate 3 162 3N KOH, 2N CZH5SOH2000K, 2N K cresylatc 2 245 4.8N KOH. 1.92N CZH5SOH2COOK, .96N K cresylate 2 245 4.17N NaOH, .83N CzHaSCHzCOONa, 2N Na cresylatc. 3 162 5.2N KOH, 2.6N CHaSOHzCOOK ll 41 4N KOH, 2N CHaSCHzCOOK, N K cresylate 3 162 3N KOH, 2N CHSSCHZCOOK, 1.5N K cresylate 2 245 4N NaOH, 2N CH3SCHzCOON8 N Na cresylate. 5. 5 86 The regeneration reaction may be accelerated by employing catalysts which have the ability to speed the oxidation of the mercaptans. For this purpose certain alkali-soluble metal oxides may be employed, such as the oxides of lead, tin, etc., but we prefer to employ compounds of the character of phenols and polyphenols in accordance with the teaching of the U. S. patent of Pevere, 2,015,038 (September 1'7, 1935). According to this method there is added to the solutizer solution a small amount of a phenol or polyphenol such as hydroquinone, catechol, pyrogallol, gallic acid, tannic acid, etc., using a concentration of about 0.1 to 1 per cent, thereby greatly increasing the rate of regeneration in the PIEBSQ I QQ 0 air 5 The efficiency of extraction is indicated by the coefficient, Kq, which is the concentration of mercaptans in the caustic solution divided by the concentration remaining in the naptha.- The use of higher concentrations of the thioether acids was found to result in separation or salting out from the solution owing to insufficient solubility. Thus 6NKOH-3N thioether acid salted out in one test. When employed in combination with cresylic acid, however, this difficulty is avoided and the distribution coefiicient is still further increased. Very little difficulty with separation and emulsification of gasoline by the alkaline solution was noted.

in countercurrent operation, it is easily possible to reduce the mercaptan concentration to a point at which the treated gasoline will react sweet to the doctor test, i. e. the standard test with sodium plumbite solution. It may often be more economical, however, to remove the major part of the mercaptans from the hydrocarbon stock by extraction with mercaptan solutizer and subsequently complete the sweetening of the stock in a separate operation with doctor solution, hypochlorite treatment, etc. to produce a stock which will pass the doctor test.

The copper number referred to above is determined by titrating a 100 ml. sample of naptha with standard ammoniacal copper solution, 1 m1. of which is equivalent to 1 mg. mercaptan sulfur per 100 ml. (1 copper number). The end point is observed when the blue color fails to be discharged after shaking.

Having thus described our invention what we claim is:

1. The process of extracting weakly acidic substances from hydrocarbon distillates which comprises contacting said distillates with a solution of an alkali metal hydroxide and a solutizer comprising an alkali metal salt of a thioether acid.

2. The process of claim'l wherein the thioether acid contains from 3 to 6 carbon atoms.

3. The process of removing mercaptans from a sour petroleum distillate which comprises contacting said distillate with a solution of an alkali metal hydroxide and a solutizer comprising an alkali metalsalt of a thioether acid.

4. The process of claim 3 wherein the said alkali metal is potassium.

5. The process of extracting weakly acidic substances from hydrocarbon distillates which comprises contacting said distillates with a solution of an alkali metal hydroxide and a solutizer comprising an alkali metal salt of an acid having the formula wherein R1 is an alkyl radical and R2 is an alkylene radical.

6. The process of removing mercaptans from a hydrocarbon distillate which comprises extracting said distillate with a solution of an alkali metal hydroxide and a solutizer comprising a mixture of alkali metal cresylate and an alkali metal salt of a thioether acid.

7. The process of removing mercaptans from a sour hydrocarbon distillate which comprises extracting said distillate countercurrently with a solution of an alkali metal hydroxide containing sufficient methioacetic acid to increase substantially the solubility of said mercaptans in said solution.

8. The process of removing mercaptans from a sour hydrocarbon distillate which comprises extracting said distillate countercurrently with a solution of an alkali metal hydroxide containing sufficient ethioacetic acid to increase substantially the solubility of said mercaptans in said solution.

9. The process of removing mercaptans from sour petroleum hydrocarbon distillates which comprises contacting the distillates with a solution containing about 1 normal to 4 normal potassium hydroxide, about 1 normal to 2 normal methioacetate of potassium and about 0.5 normal to 1.5 normal potassium cresylate.

10. In the process of removing mercaptans from a sour petroleum distillate wherein said distillate is contacted with a solution of an alkali metal hydroxide and a mercaptan solutizer, the solution containing dissolved mercaptans is separated from said distillate regenerated by the action of an oxygen-containing gas, whereby the said mercaptans are converted to disulfides insoluble in said solution, the resulting disulfides are separated from the solution and the solution is returned to the extraction step for the removal of mercaptans from additional amounts of petroleum distillate, the improvement comprising employing as the solutizer in said alkali metal hydroxide solution an alkali metal salt of a thioether acid.

CHESTER E. ADAMS. THEODORE B. TOM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,186,398 Yabrofi Jan. 9, 1940 2,212,106 Yabroff Aug. 20, 1940 2,381,859 Ayers et al Aug. 14, 1945 2,369,771 Bond Feb. 20, 1945 2,273,104 Hellman Feb. 17, 1942 2,015,038 Pevere Sept. 17, 1935 

